Building a Tank Within a Tank

Sierra Madre, Calif., challenged with trying to replace an existing water storage tank with two new tanks while having only one piece of land available

The city of Sierra Madre, Calif., was recently challenged with trying to replace an existing water storage tank with two new tanks and having only one piece of land available for the site.
In order to achieve this goal, two ultimate goals had to be met. First, the project had to comply with the local health department’s preference for multiple reservoirs within each pressure zone—only one reservoir served Zone 2 at the time. Secondly, the one small piece of land available had to be used for constructing two reservoirs for the project where only one would fit.
In order for the city to deliver a precious resource and make efficient use of the land while answering both the needs of the community and the city, the solution was to build a 2.8 million-gallon water storage tank-within-a-tank.
To make this vision a reality, the city hired a project team comprised of RBF, consulting engineer; SSC Construction, Inc., general contractor; Bucknam and Associates, project manager; and DYK, Inc., the tank prestressor.

Project parameters

The Grove Street Reservoir Project involved the removal of
an existing reinforced concrete tank and the construction of a prestressed concrete tank with a concentric concrete inner wall. The site was extremely tight and completely surrounded by existing residential development. The maximum setback from the property line on any side of the reservoir was only 28 ft.
The Grove Street Reservoir was the only reservoir in the city’s Zone 2, serving approximately 33% of the city’s residents. The available land was the only space available for construction of a water storage tank.
Because the city did not have the option to build an additional reservoir within this Zone and the existing site could not support separate tanks, a concentric tank inside an outer tank concept was adopted. The tank-within-a-tank concept satisfied the Health Department’s preference for reservoir separation while accommodating two tanks on one site.
The outer walls of the water tank were both circumferentially and vertically prestressed by DYK, Inc. Meanwhile, vertical post-tensioning was achieved with 126, 13/8-in., diameter high-strength steel vertical tendons.
Circumferential prestressing was completed using 7-wire galvanized strand pulled to 14,950 lb and protected with several coats of automatically applied shotcrete.
The prestressing machine continuously and electronically monitored the applied stressing force on the strand as it was applied. By keeping the corewall in compression along with the independent connections, it ensures a long life water tight structure.
Another significant aspect of the tanks that make the reservoir unique is the concentric tanks are normally operated in tandem, with water levels raising and lowering in synchronization. However, the design of these two tanks allows for the tanks to be operated independently. Consequently, this provides each tank the ability to be drained for cleaning or inspection.
Innovative design and construction techniques were incorporated into the Grove Street Reservoir Reconstruction project to provide the community of Sierre Madre with safe and reliable water storage while ensuring efficient use of available land.